In the process of aluminum production, alumina is dissolved in cryolite in electrolytic cells, or pots, which are steel shells lined with carbon. A number of pots, usually more than 100, are arranged in series to form a potline. An aluminum production plant may have several potlines. The pots contain a molten electrolyte consisting primarily of cryolite (Na.sub.3 AlF.sub.6) and operate at approximately 930.degree. to 1000.degree. C. Other materials are added to the electrolyte to improve the efficiency of the operation or to reduce power consumption, such as alumina, aluminum fluoride, sodium fluoride, soda ash, calcium fluoride, lithium carbonate and magnesium oxide.
The hearth or lining of the cell is composed of carbon, which is backed by insulation and contained within a steel container called a potshell. The carbon portion of the lining serves as the cathode and contains the molten electrolyte. The carbon lining is composed of prefabricated carbon blocks joined together by a carbon paste, which is hydraulically rammed in the seams between the carbon blocks. The sidewalls of the lining are typically formed with carbon paste, but may contain prefabricated carbon blocks. The carbon material within the lining, both blocks and paste, is predominantly anthracite-based material. It may contain some graphite to improve its electrical and thermal properties. Insulation packages for a cell are mostly of two types, a loose bed of alumina powder or refractory brick and/or castable.
Over the life of the cathode and its cell lining, the carbon and insulating materials become impregnated with fluoride-containing salts. As the ingress of salts continues, the integrity of the lining is adversely affected. Sodium, in particular, can actually intercalate within the crystalline lattice of the carbon materials, causing distortion and stresses within the lining. The insulating materials become more thermally conductive as they are impregnated by these fluoride salts. Failure can occur by cracking or excessive heaving of the lining. When these failures occur, the cell is taken off-line and the cathode lining material is removed from the potshell by mechanized digging equipment This spent cathodic material is referred to as spent potliner (SPL). The life cycle of a cathode can be from about three to about ten years. Since there are numerous pots located at a single aluminum reduction plant, the decommissioning and relining of cathodes is a continual process.
In addition to containing fluoride salts, as mentioned above, SPL contains cyanides that are formed by the ingress of air through openings in the potshell and subsequent reaction of nitrogen with the carbon lining. Therefore, cyanide is concentrated around the perimeter of the cathode, predominantly in the rammed end walls and sidewalls of the carbon lining. As the size, or capacity, of the cell is increased, the ratio of the mass around the cell periphery to the total mass of the cathode lining is reduced, and the concentration of cyanide in the spent potliner is reduced accordingly. Therefore, the concentration of cyanides in older, smaller, and more air-permeable cathodes is greater than in larger, more modern cells.
The major types of insulation used in cathodes are metallurgical alumina, having a loose, sandy appearance, and refractory bricks. Depending on the type of brick used, spent potliner may contain oxides of silicon which would not be present in cathodes insulated with alumina. The oxides do not pose an additional environmental liability regarding spent potliner and are essentially inert in the process for treating or disposing of spent potliner. The brick-insulated cathodes are also less prone to absorb fluoride salts, in which case the soluble fluoride content would be lower.
Spent potliner contains a small amount of semi-volatile organics which, in all probability, originate from the carbon paste used to fill the seams between the cathode blocks and to form the cathode sidewalls and end walls. The carbon paste forming the outermost part of the sidewalls, and thus close to the steel potshell, is not baked to sufficiently high temperatures during cell operation to carbonize all of the pitch used as a binder in the paste. A small portion of the semi-volatile organics then remain in the spent potliner. Again, the amount present can be related to the physical size of the cathode. The larger and thicker the sidewalls, the less likely that the outermost paste will be completely baked.
Spent potliner was listed by Environmental Protection Agency (EPA) on Sep. 13, 1988 (53 Fed. Reg. 35412) as a hazardous waste (K088) under 40 C.F.R., Part 261, Subpart D because it may contain significant amounts of iron cyanide complexes and free cyanide These recent actions create an immediate need in the aluminum industry for an economical process for detoxifying spent potliner such that the treated residue is not a hazardous waste. This is important because of the need for alternatives to land disposal of hazardous waste, established as national policy in the RCRA Hazardous and Solid Waste Amendments (HSWA) of 1984, and the anticipated lack of hazardous waste treatment capacity.
A review of the literature shows the composition of SPL to be highly variable. The range of analyses is given in Table I. Any process for the treatment of SPL must be versatile enough to treat SPL generated while using different cell designs, electrolyte compositions, and insulation packages, and any residues generated must meet anticipated EPA-defined limits for all constituents of concern (e.g., cyanide, fluoride, organics and metals). The components of SPL of greatest concern environmentally are cyanide and soluble fluoride salts.
TABLE I ______________________________________ Spent Potliner Constituents Component Range of Compositions, % ______________________________________ C 9.6-51.0 Na 7.0-20.0 Al 4.7-22.1 F 9.7-18.9 Ca 1.1-2.9 Li 0.3-1.1 Mg 0.3-0.9 Si 0.0-12.3 Fe 0.3-2.1 S 0.1-0.3 CN 0.02-0.44 ______________________________________
The aluminum industry has long recognized the environmental liability of SPL and is pursuing many options for treatment and/or disposal. These options include landfilling, recycling as a feedstock in other industries, such as the steel, cement, aluminum, or mineral wool industries, fluidized bed combustion, cryolite recovery, pyrohydrolysis, pyrosulfolysis, and others. Landfilling is an option that is presently available but will become increasingly expensive, since hazardous waste landfills are required. Recycling through other industries is an attractive and proven option, however, the classification of SPL as a hazardous waste will greatly discourage other industries from utilizing SPL, due to the cumbersome and expensive environmental regulations. Some of the other technologies may eventually have application, but many involve excessive cost and have never been proven on an industrial scale.